1. Field of the Invention
The invention relates to alkali-free aluminoborosilicate glasses. The invention also relates to uses of these glasses.
2. Background of the Invention
High requirements are made of glasses for applications as substrates in flat-panel liquid-crystal (or expressed differently: liquid crystal) display technology, for example in TN (twisted nematic)/STN (supertwisted nematic, or expressed differently: super twisted nematic) displays, active matrix liquid crystal displays (AMLCDs), thin-film transistors (TFTs) or plasma addressed liquid crystals (PALCs). Besides high thermal shock resistance and good resistance to the aggressive chemicals employed in the process for the production of flat-panel screens, the glasses should have high transparency over a broad spectral range (VIS, UV) and, in order to save weight, a low density. Use as substrate material for integrated semiconductor circuits, for example in TFT displays (“chip on glass”) in addition requires thermal matching to the thin-film material silicon which is usually deposited on the glass substrate in the form of amorphous silicon (a-Si) at low temperatures of up to 300° C. The amorphous silicon is partially recrystallized by subsequent heat treatment at temperatures of about 600° C. Owing to the a-Si fractions, the resulting, partially crystalline poly-Si layer is characterized by a thermal expansion coefficient of α20/300≅3.7×10−6K. Depending on the a-Si/poly-Si ratio, the thermal expansion coefficient α20/300 may vary between 2.9×10−6/K and 4.2×10−6/K. When substantially crystalline Si layers are generated by high temperature treatments above 700° C. or direct deposition by CVD processes, which is likewise desired in thin-film photovoltaics, a substrate is required which has a significantly reduced thermal expansion of 3.2×10−6/K or less.
In addition, applications in display and photovoltaics technology require the absence of alkali metal ions. Sodium oxide levels of less than 1000 ppm (parts per million) as a result of production can be tolerated in view of the generally “poisoning” action due to diffusion of Na+ into the semiconductor layer.
It should be possible to produce suitable glasses economically on a large industrial scale in adequate quality (no bubbles, knots, inclusions), for example in a float plant or by drawing methods. In particular, the production of thin (<1 mm) streak-free substrates with low surface undulation by drawing methods requires high devitrification stability of the glasses. Compaction of the substrate during production, in particular in the case of TFT displays, which has a disadvantageous effect on the semiconductor microstructure, can be countered by establishing a suitable temperature-dependent viscosity characteristic line of the glass: with respect to thermal process and shape stability, it should have a sufficiently high glass transition temperature Tg, i.e. Tg>700° C., while on the other hand not having excessively high melting and processing (VA) temperature, i.e. a VA of ≦1350° C.
The requirements of glass substrates for LCD display technology or thin-film photovoltaics technology are also described in “Glass substrates for AMLCD applications: properties and implications” by J. C. Lapp, SPIE Proceedings, Vol. 3014, invited paper (1997), and in “Photovoltaik—Strom aus der Sonne” by J. Schmid, Verlag C. F. Müller, Heidelberg 1994, respectively.
The abovementioned requirement profile is fulfilled best by alkaline earth metal aluminoborosilicate glasses. However, the known display or solar cell substrate glasses described in the following publications still have disadvantages and do not meet the full list of requirements:
Numerous documents describe glasses having low MgO contents: JP 9-169 538 A, JP 4-160 030 A, JP 9-100 135 A, EP 714 862 A1, EP 341 313 B1, U.S. Pat. No. 5,374,595, WO 97/11919 and WO 97/11920. Such glasses, in particular those of EP 714 862 A1 and JP 9-169538 A, do not have the desired meltability, as is evident from very high temperatures at viscosities of 102 dPas and 104 dPas, and have a relatively high density. The same applies to the MgO-free glasses of DE 37 30 410 A1.
The glasses of U.S. Pat. No. 5,374,595 have high BaO contents of 2–7 mol % which leads to undesirably high densities of these glasses. The same applies to the glasses of JP 61-132536 A, JP 8-295530 A, JP 9-48632 A and JP 9-156953 A.
Similarly, the glasses of JP 10-72237 A having high SrO contents have very high temperatures at viscosities of 102 dPas and 104 dPas, as is evident from the examples.
The same is true for glasses having low B2O3 contents as described in JP 9-263421 A and JP 10-45422 A. The devitrification tendency will be disadvantageously high, in particular in combination with low BaO contents. On the other hand, excessively high B2O3 contents—such glasses are described, for example, in U.S. Pat. No. 4,824,808—are disadvantageous for the intended properties of high heat resistance and high chemical resistance, in particular to hydrochloric acid solutions.
Low-SiO2 glasses do not have sufficient chemical resistance either, in particular when they contain relatively large amounts of B2O3 and are low in alkaline earth metals. This applies to the glasses of WO 97/11919 and EP 672 629 A2. The relatively SiO2-rich variants of the latter document have only low Al2O3 levels, which is disadvantageous for the crystallization behavior.
JP 9-123 33 A, which relates to glasses for hard disks, describes compositions of SiO2, Al2O3, CaO and further optional components including B2O3. The glasses listed have high alkaline earth metal oxide contents and thus have high thermal expansion, which makes them unsuitable for use in LCD or PV technology. Their visual quality will probably also be inadequate.
Federal Republic of Germany Patent No. 196 17 344 C1 (U.S. Pat. No. 5,908,703) and Federal Republic of Germany Patent No. 196 03 698 C1 (U.S. Pat. No. 5,770,535) by the Applicant disclose alkali-free, tin oxide-containing, low-SiO2 or SrO-free glasses having a coefficient of thermal expansion α20/300 of about 3.7·10−6/K and very good chemical resistance. They are suitable for use in display technology. However, since they must contain ZnO, they are not ideal, in particular for processing in a float plant. In particular at higher ZnO contents (>1.5% by weight), there is a risk of formation of ZnO coatings on the glass surface by evaporation and subsequent condensation in the hot-shaping range.
WO 98/27019 describes glasses for display and photovoltaics applications having a low density and a high heat resistance. In these glasses, some of which have a high CaO content, the SrO and BaO contents are limited to a total of 3% by weight, which renders the glasses susceptible to crystallization.
DE 196 01 022 A1 describes glasses which are selected from a very wide composition range and which must contain ZrO2 and SnO. The glasses, which, according to the examples, have a relatively high BaO content, tend to exhibit glass defects because of the ZrO2 level which has to be present.
DE 42 13 579 A1 describes glasses for TFT applications having a coefficient of thermal expansion α20/300 of <5.5×10−6/K, according to the examples of ≧4.0×10−6/K. These glasses which have relatively high B2O3 levels and relatively low SiO2 contents do not have a high chemical resistance, in particular to diluted hydrochloric acid.
In the unexamined Japanese publications JP 10-25132 A, JP 10-114538 A, JP 10-130034 A, JP 10-59741 A, JP 10-324526 A, JP 11-43350 A, JP 11-49520 A, JP 10-231139 A and JP 10-139467 A, mention is made of very wide composition ranges for display glasses, which can be varied by means of many optional components and which are admixed with one or more specific refining agents in each case. However, these documents do not indicate how glasses having the complete requirement profile described above can be obtained in a specific manner.